Guild Wars 2

For example, if I wanted 800 gems, how much gold would I need in average?
Please answer as soon as possible!

You can check the average conversion rates on the TP.
Right now its about 11.5g/100 gems so 92g/800 gems

When I looked earlier, out of curiosity, it would have cost me over 100g for 800 gems.

The rate is constantly in flux and tends to go up over time.

http://www.gw2spidy.com/gem

Over the last 24 hours it was between 85 and 100 gold.